Recent years have seen significant developments in high-performance devices exploiting superior properties of infrared rays. For example, applications relying on the sensing capability of the rays include a surface thermometer that measures the surface temperature of an object on a noncontact basis, a resources survey system that senses the distribution of the resources on the earth from high in the universe, a device that detects an object in a dark field, a security system that detects a human body, and gas-analyzing equipment. There are still other examples such as an infrared image-processing device that processes collected data into images and a high-power laser processing device utilizing the thermal energy of infrared rays.
The increased applications of such high-performance devices using infrared rays have caused a growing demand for enhanced performance and a decrease in the production cost of the optical components, such as windows and lenses, to be incorporated into these devices.
Inorganic materials such as germanium (Ge), zinc selenide (ZnSe), and zinc sulfide (ZnS) have been mainly used as the materials for the optical. components for a wavelength range of 8 to 12 μm. Polyethylene and other plastic materials, which are less costly and have good processibility, also have been used for the same purpose in recent years. The inorganic materials have occasionally revealed their insufficient properties when used in outdoor-type devices, vehicle-mounted devices subjected to vibration, thumps, and bumps, or other devices exposed to severe operation environments. More specifically, when used singly, they lack in mechanical strength, surface hardness, and resistance to surface oxidation by ultraviolet rays, depending on the operating conditions. To improve their properties, the surface of the inorganic materials has been coated with an environment-resistant layer in some instances. For example, the published Japanese patent application Tokukaishoh 56-87002 has disclosed an optical component whose surface is coated with a diamond-like carbon layer transparent to infrared rays. This method, however, increases the production cost considerably.
On the other hand, when a plastic optical component is used, not only the heat resistance but also the mechanical strength as an entire optical component is unavoidably reduced in comparison with the inorganic materials. For example, to increase the transmittance of a plastic optical component, it is necessary to reduce the total thickness of the component. This thickness reduction inevitably reduces the mechanical strength.
The materials for these optical components usually transmit rays in a wide range of wavelengths from visible rays to infrared rays. Consequently, when a system is designed to selectively detect infrared rays, 8 to 12 μm in wavelength, emitted from the human body, the other rays outside the detecting wavelength range become noise. A typical noise is visible and near-infrared rays from 0.4 to 3 μm in wavelength. The noise causes malfunction in the signal treating section after the signal detection or reduces the detection accuracy due to the rise in the background level.
To block such a noise, a conventional measure is to form a noise-blocking filter layer on the surface of the main body of an optical component. However, the formation of such a filter layer is costly because the layer is formed by the vapor phase deposition method, such as the sputtering, vacuum deposition, or chemical vapor deposition (CVD) method. Consequently, researchers and engineers have been required to develop an optical component that has sufficient transmittance for infrared rays in a specified wavelength range, that can reliably block visible and near-infrared rays, and that can be produced at low cost. More specifically, the material for the component is required to have the lowest possible transmittance for rays having wavelengths ranging from 0.4 to 3 μm and the highest possible transmittance for rays having wavelengths ranging from 8 to 12 μm.
To block visible and near-infrared rays, studies have been done on developing a means in which particles are dispersed in the main body of an optical component to selectively absorb visible and near-infrared rays. In this case, plastic materials are mainly used as the base material. For example, another published Japanese patent application, Tokukaishoh 61-39001, has disclosed an optical component in which a plastic material such as high-density polyethylene is used as the base material to disperse inorganic pigments such as titanium oxide (TiO2), barium sulfate (BaSO4), red iron oxide (Fe2O3), magnesium oxide (MgO), and zinc (Zn). This component, however, is insufficient in blocking noise-causing rays having wavelengths ranging from 1 to 2 μm and therefore is unsuitable for a lens to be used in a sensor that selectively detects infrared rays having wavelengths of 3 μm and more. Yet another published Japanese patent application, Tokukaishoh 62-284303, has disclosed an optical component in which a similar plastic material is used as the base materiel to disperse a zirconium (Zr) compound for selectively transmitting rays having wavelengths ranging from 7 to 14 μm. However, this material requires the dispersion of a 5 to 15 wt. % Zr compound to sufficiently block of noise. The result is a significant reduction in the transmittance of infrared rays.
To solve the above-described problems, the published Japanese patent application Tokukaihei 9-21701 has disclosed an optical component in which a similar plastic material is used as the base material to disperse at most 4 wt. % ZnS in the form of fine particles. Another published Japanese patent application, Tokukouhei 7-86566, has disclosed an optical component in which a similar plastic material is used as the base material to disperse fine particles of pigments such as iron tritetraoxide (Fe3O4) particles, carbon black, titanium oxide (TiO2) particles coated with a tin oxide (SnO2) layer, and zirconium oxide (ZrO2) particles. Yet another published Japanese patent application, Tokukaihei 8-54478, has disclosed that it is desirable to use zinc selenide (ZnSe) as the pigment to be dispersed in a similar plastic material for a lens to selectively block near-infrared rays. However, the property of blocking visible and near-infrared rays and the property of transmitting far-infrared rays have a mutually contradictory relationship. Because of this complexity, these methods disclosed so far are incapable of producing a material for an optical component that satisfies both requirements concurrently with a desirable balance. On the other hand, a dense ZnS sintered body has a good transmitting property in a wide infrared range of 1 to 14 μm. Exploiting this property, the published Japanese patent application Tokukaihei 11-295501 has disclosed a polycrystalline ZnS sintered body having a capability to block visible and near-infrared rays and having an improved property of transmitting far-infrared rays, and a method for producing the sintered body as well. The sintered body is materialized by controlling its porosity. The porosity-controlling method apparently blocks visible rays by scattering them. When the ZnS sintered body is used as a window or lens for a sensor, if the distance between the sensor and the window or lens is large, visible rays can be blocked. However, if the distance is small, the scattered rays can be detected as noise.